1. Field of the Invention
The present invention relates to a displacement correction device and a method thereof, and an image forming device preferably used in a tandem-type color image forming device where a plurality of image stations per a color that execute an image forming process are provided and a color image for a transfer formed by superimposing images of each color on an endless transferring belt or a transporting belt is transferred onto a recording paper, thereby forming the color image.
2. Description of the Related Art
Conventionally, in a color image forming device employing an electrophotographic technology, a photoconductor drum as an image holding member is electrically charged by a charging unit, a latent image is formed on the charged photoconductor drum by irradiating the surface of the charged photoconductor drum with laser light depending on image information, and a developing unit develops the latent image. The developed toner image is then transferred onto a paper sheet or the like.
Tandem-type color image forming devices are well-known in the art. In a typical tandem-type color image forming device includes a plurality of image stations that execute such a series of image forming processes. Each of the image stations forms an image of a corresponding color of C (cyan), M (magenta), Y (yellow), and BK (black) on a corresponding one the image holding member and the color image for a transfer formed by superimposing images of each color on an endless transferring belt is transferred onto a recording paper, thereby forming the color image.
In this tandem-type color image forming device, when the image is formed on the photoconductor drum of each color and transferred onto the recording paper on the transferring belt, if a transferred image position of each color is displaced from an ideal position, the image having a color drift is formed and quality of the image deteriorates.
Generally, a displacement correction pattern image is formed on the transferring belt to be detected by a charged coupled device (CCD) sensor or the like and the displacement on the photoconductor drum corresponding to the image of each color is detected and an electric correction is performed to an image signal to be recorded, and a reflecting mirror provided in a light path of the laser light is driven to correct a light path length or a light path change. Japanese Patent No. 2642351 discloses a method for achieving this. The displacement correction pattern image is a line-shaped pattern of a toner in each color of Y, M, C, and BK. This method includes measuring a time when the pattern of other color is detected by the sensor, by making a pattern of one certain color a reference, and calculating how much a position of other pattern calculated from the time and transporting speed of the transporting belt is displaced from a theoretical value to obtain the displacement amount.
Japanese Patent Application Laid-Open No. H6-193476 discloses a method to image-form plural sets of the patterns-for-displacement-correction in a sub-scanning direction (in a longitudinal direction) of the transporting belt relative each color and to average the displacement amount per a set, thereby eliminating fluctuation of the displacement amount due to a rotation periodic fluctuation of the photoconductor drum. In this method, however, the primary time cannot be inserted between the sets, due to environmental changes or a tolerance in a layout or in a dimension of a portion where the displacement correction pattern is image-formed, detected in a design phase, for example, a stretch of the transferring belt, an incident sometimes occurs where positional information on the correction pattern cannot be detected.